Deciding on the best weapon loadout for you is understandably a difficult one since the introduction of MK2 weapons and ammo. There are many combinations you could use based on whether you like to play more against other players (e.g. Deathmatches) or NPCs.
MK2 Ammo and the weapons they go on are often misunderstood parts of GTA Online. MK2 Weapons and Ammo are superior and more expensive than the standard counterparts.